1. Technical Field of the Invention
This invention relates generally to a vinylidene fluoride polymer and a method of making the same, and more particularly to a vinylidene fluoride polymer (hereinafter referred to as "PVDF" in short) and a method of manufacturing the same with a suspension polymerization process.
2. Description of the Prior Art
PVDF is now being widely used as a material for piping and valves used in chemical plants, the lining and coating materials of storage tanks and reaction vessels due to its excellent mechanical strength and chemical resistance. PVDF has such good melt-processability and thermal stability that it can be processed without recourse to thermal stabilizers, processing aids, etc. Due to this unique features, PVDF moldings have a feature of high purity. Its high purity, together with its excellent chemical resistance, often makes PVDF particularly suitable for materials for facilities for the manufacture and storage of ultra-pure water and ultra-pure chemicals used in semiconductor manufacture.
PVDF is commercially manufactured by the suspension polymerization and emulsion polymerization processes. The suspension polymerization process is such that monomers are dispersed in droplets in the water as a suspension medium using a dispersant, and are polymerized using organic peroxides dissolved in the monomers to obtain granular polymers of sizes from 100 to 300 microns. Suspension polymerization products are manufactured with a simpler process, and easier to handle because of their granular properties, compared with emulsion polymerization products. In addition, suspension polymerization products are of higher purity than emulsion polymerization products because they do not contain emulsifiers or salting agents. Formation of large spherulites in moldings of suspension polymerization products, on the other hand, may be regarded as a drawback in some applications.
PVDF is a crystalline resin which tends to form spherulites during cooling and solidification after melt processing, and it is generally known that the surface smoothness of PVDF moldings largely depends on the size of spherulites.
When PVDF having lowered surface smoothness due to the presence of large spherulites is used to manufacture piping for ultra-pure water manufacturing lines, minute dents between the spherulites on the inside surface of the PVDF piping act as a source for propagation of microorganisms, lowering the purity of pure water. When used as coatings for reaction vessels and storage tanks, the PVDF paint films having large spherulites are susceptible to stress cracking, reducing their durability. It is generally believed that the size of spherulites depends on the cooling rate of PVDF moldings after melt processing; the faster PVDF moldings are cooled, the finer spherulites are formed. In some molding methods, quenching may be impossible. During the extrusion of thick-wall pipe, for example, when the extruded pipe is cooled from the external surface, large spherulites are formed because the inside of the pipe is not cooled so quickly, deteriorating the smoothness of the pipe inside. To overcome this problem, therefore, polymers having such crystallization characteristics as to form fine spherulites easily even at a relatively low cooling rate have long been needed.
Means for reducing the size of PVDF spherulites include;
(1) The size of spherulites of PVDF moldings can be reduced to some extent by increasing the molecular weight of polymers. With this method, however, satisfactory effects cannot necessarily be accomplished. Conversely, as the molecular weight is increased, some undesirable phenomena, such as the difficulty in processing due to increased melt viscosity, discoloring caused by the decomposition of polymers due to increased processing temperature, and formation of hydrogen fluoride due to increased processing temperature, are encountered.
(2) As proposed in U.S. Pat. No. 3,701,749 and U.S. Pat. No. 3,719,644, the spherulites of PVDF moldings can be reduced in size by adding a nucleating agent, such as flavanthrone and salt. Addition of these nucleating agents is not favorable because it could lower the thermal stability of PVDF, and lead to discoloring caused by the decomposition of polymers, as well as to the formation of hydrogen fluoride. Furthermore, these nucleating agents are impurities in polymers, making the polymers unsuitable for applications requiring high purity, such as ultra-pure water piping.
(3) It is known that the size of spherulites can be reduced by copolymerizing vinylidene fluoride monomers with tetrafluoroethylene, trifluoromonochloroethylene, hexafluoropropylene, vinyl fluoride, etc. To sufficiently reduce the size of spherulites with this method, however, more than 10 wt. % of comonomers are required. As a result, the degree of crystallization of polymers is lowered, and the crystalline melting point is also lowered remarkably. This leads to deterioration in heat resistance, chemical resistance and mechanical strength as beneficial properties of PVDF.
(4) U.S. Pat. No. 3,798,287 has proposed a method of adding fluorine monomers which give the polymer having higher crystallization temperatures than PVDF after the polymerization of vinylidene fluoride monomers has been completed so as to facilitate polymerization within PVDF particles. With this process, however, vinyl fluoride severely reduces the thermal stability of the resulting PVDF. Moreover, post-addition of trifluoromonochloroethylene has less effects of reducing the size of PVDF spherulites, and also lowers the thermal stability of PVDF.
Post-addition of tetrafluoroethylene, on the other hand, remarkably reduce the size of spherulites and improve the thermal stability of PVDF, but polytetrafluoroethylene hardly disperse uniformly in PVDF, making the size of the spherulites of PVDF moldings uneven, and posing some problems in the surface smoothness of moldings.